1. Field of the Invention
The present invention relates to synthetic branched polyisoprenes having macrostructural and microstructural characteristics very similar to those of natural rubber, and to a synthesis process for these polyisoprenes. These synthetic polyisoprenes may advantageously be used in rubber compositions for tires as a replacement for natural rubber.
2. Description of Related Art
Natural rubber (also referred to as “natural polyisoprene”) is still widely used in rubber compositions for tires, despite the advances made in recent years in obtaining synthetic polyisoprenes capable of partially replacing natural rubber in certain locations in tires.
Since natural rubber is in particular characterised in the pure state by a cis-1,4 linkage content of 100%, attempts have been made to synthesise polyisoprenes which have a value for said content which is as close as possible to 100%.
The most significant advance in this area is described in International Patent Specification WO-A-02/38635 in the name of the applicants, which proposes a catalytic system of the “preformed” type based on at least:
a conjugated diene monomer,
an organic phosphoric acid salt of one or more rare earth metals (metals with an atomic number between 57 and 71 in Mendeleev's periodic table of elements), said salt being in suspension in at least one inert, saturated and aliphatic or alicyclic hydrocarbon solvent,
an alkylating agent consisting of an alkylaluminum of formula AlR3 or HAlR2, the (alkylating agent:rare earth salt) molar ratio ranging from 1 to 5, and
a halogen donor consisting of an alkylaluminum halide.
This catalytic system makes it possible to polymerise isoprene with satisfactory activity at polymerization temperatures which are less than or equal to 5° C., and to obtain polyisoprenes at these low temperatures which have cis-1,4 linkage contents, measured both by the carbon 13 nuclear magnetic resonance method and by mid-infrared analysis, which are strictly greater than 99.0%.
Natural rubber is also characterised in that, when uncrosslinked (i.e. in the “uncured” state before any curing has been carried out), it exhibits a force-elongation curve which is very much “straightened out” relative to those of hitherto known synthetic polyisoprenes (i.e. the respective gradients of the tangents to this curve at given relative elongations are much steeper for natural rubber, for example at relative elongations of 300% and 400%), which means that, when tension is applied thereto, natural rubber crystallises much more in the uncrosslinked state than do synthetic polyisoprenes.
This inadequate crystallisation under tension exhibited by synthetic polyisoprenes in particular means that the mechanical properties of rubber compositions incorporating them sometimes degenerate greatly during processing, which makes these compositions unsuitable for replacing those based on natural rubber at all tire production stations.
Natural rubber also differs from hitherto known synthetic polyisoprenes with regard to its degree of branching in the uncrosslinked state, said degree being very high in the case of natural rubber, but relatively low in the case of synthetic polyisoprenes.
One major drawback of hitherto known synthetic polyisoprenes is therefore that, in the uncrosslinked state, they do not exhibit the above-stated characteristics of crystallisation under tension and of branching which in particular characterise natural rubber.